Aluminum etch



Patented May 5, 1953 Wichita, Kane... assignors to Beech Aircraft: Corporation, Wichita, Kansl, a corpor-ationv of Delaware No Drawing. Application February 12,1952; Serial N0. 271,292

(01.. ale-4' 1 Claim. 1

The invention disclosed in this patent relates to the; etching and cleaning of aluminum alloyspreparatory" to shielded arc, welding or to elec trical resistance welding,such asspotwelding-and Objects of the invention are to elfect rapid, uniform chemical cleaning in a single dip operation, at room temperature and under generally practical and economical operating conditions.

Special objects of the invention are to insure accomplishment of better, stronger welds, faster welding operations, a greater number of welds between tip cleanings, consistency of welds at prescribed machine settings, top quality welds of shielded arc weld machines, less down time for cleaning electrode tips and machine adjustments, and generally improved welding conditions.

The invention is particularly directed toward the treatment of more refractory aluminum alloys associated with aircraft, epecially that identified as 618. The heat treating operation on this particular aluminum alloy has the effect of creating a dense refractory coating, difficult of removal by known etching operations.

It has been a purpose of the present invention to provide an etch which would remove such dense refractory coatings without injuring the metal surface and which would leave the surface chemically clean, smooth and hard, ready for uniform spotwelding and inert gas shielded arc welding operations.

Other desirable objects attained by the invention and the novel features constituting the invention are set forth and will appear in the course of the following specification.

In developing an etch which would accomplish the foregoing, it was determined that the fluoride ion, in some combination, was essential to satisfactory results.

Commercial grades of hydrofluoric acid (48- 52%) may be used. Dilute solutions of hydrofluoric acid vigorously attack aluminum alloys but in addition cause deposition of a thin, loosely adherent layer of a black to brownish-black smut.

Addition of nitric acid, up to about or more by volume, to an approximate 3% by volume, hydrofluoric acid solution constrains or restrains the vigorous action of the fluoride ion and to an extent retards the smut formation.

Addition ofboric acid tosthenitricrhydrofluorica combinationeffectsa still further leveling action and eliminatesathe smut formation.

The amount of boric acid, either powder or crystals, may be approximately 1 by wei'ght'.

This combined nitric-hydrofluoric-boric acid combination in the approximate ratios stated, is especially effective in removing the dark, dense, refractory coating resulting from the heat treating operation on 618 aluminum alloy.

A single clip with an immersion time of four minutes has been found ample even for the most refractory parts.

This operation may be carried on at normal room temperatures and even ranging as high as 130 F.

The cost per unit treated is low and the solution may be rejuvenated and balanced as it is expended through usage.

The corrosive character of the solution requires care in handling. Tank linings of Koroseal and carbon brick may be used and effective surface exhaustion of vapors and gases evolved, may be employed.

Examples of properly balanced and buffered solutions necessary for smooth, bright, hard, smut-free surfaces of aluminum alloy products, are given as follows:

By i iiii c iiidig z r.1.42,20% by volume Hydrofiuoric acid cone), 3% by volume Boric acid (powdered or crystal), 1% by weight Water to make 100% by volume 2. By volume and weight figures To make 3,000 gallons Nitric acid, 42 B. (67-68% HNOs), 600

Norm 1.-When hydrofluoric acid is used instead of the 50% type, a total of only gallons will be required to supply the correct fluoride concentration.

Nora 2.Secure complete solution of the boric acid before etching any parts in the tank.

Cooling water may be run through a jacketed section of the tank to keep the solution operating at room temperatures, say up to F., and the action may be governed to avoid going above such temperature.

The invention is particularly suited to conveyorized operations, with the material or objects transported by conveyor through the tank at a rate for approximately four minutes immersion. 1

Various concentrations of acid may be used, but the approximate ratio of nitric acid to hydrofluoric acid to boric acid given should be maintained.

Rejuvenation may be effected at regular periods as by addition, say, of one-third each of the original chemicals, such as:

Nitric acid /3X600=200 gallons Hydrofluoric acid 90: 30 gallons or,(if 60% sol. used) '75: 25 gallons Boric acid /3X280: 93 pounds A measure of the quality of the chemical cleaning effected may b determined by the surface resistance as expressed in microhms.

The etch described gives surface readings of or less microhms, and this will give 150 or more spots, meeting all specification requirements, between tip cleanings, and much higher production than possible with currently available etches.

Consistency attained in the pretreatment described results in consistency of welds at prescribed machine settings. The solution made up in the ratios disclosed is so balanced as to yield surfaces having consistent microhm resistance readings, enabling greatly increased number of spotwelds between tip cleanings.

Greater economy of operation also results from the fact that it may not be necessary, for chemical reasons, to discard an entire batch of etch solution, because the drag-in and drag-out plus the additions of make-up permits practically continuous operation with the solution until such time as possibly mechanical failure or maintenance reason may necessitate shut-down and complete dumping of a batch.

While particularly useful for 618 aluminum alloy, the invention, it will be realized, is adapted to the treatment of various other aluminum alloys.

What is claimed is:

Aluminum etch comprising the following in approximately the proportions:

Nitric acid, 42 B 20 gallons Hydrofluoric acid (48-52% HF) 3 gallons Boric acid 9.3 pounds Water to make gallons total.

ROSCOE D. HOWARD, ROBERT E. SAMUELSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,409,271 Goldowski Oct. 15, 1946 2,465,750 Reid Mar. 29, 1949 2,593,447 Hesch Apr. 22, 1952 

